I'd like to buy Algodoo for my kids. However, before making the purchase, I'd like to clarify some questions regarding the licensing.

The thing is, the kids might use PC either at home, or at country house. That means, to different PCs. I am not sure how the activation of Algodoo exactly happens - do I get a unique key I must enter in the demo version? If so, will the same key be accepted also on the other PC? Or maybe it is considered that I should buy two licenses for such 2-PC setup?

From what I've heard, it'll automatically unregister on the first computer if you do that, but it's never done that as far as I know. You can do this without breaking the ToS if you uninstall it on the first computer before you install it on the second, though.